1. Field of the Invention
This invention relates to a power line communication MODEM for transferring data from an information processing unit such as a personal computer through a power line.
2. Description of the Related Art
Power lines include a distribution line for sending power from a substation to each home and an interior distribution line extended inside home. A power line communication MODEM (abbreviated as “PLC MODEM”) is a device that makes it possible to use these power lines as a communication line. The PLC MODEM converts digital data from an information processing unit such as a personal computer to analog signals, overlaps the signals with the power line of a commercial power source, converts the analog signals inputted through the power line to the digital signals and sends them to the information processing unit. Therefore, the PLC MODEM can constitute a communication network among the information processing units.
FIG. 5 is a perspective view of a known PLC MODEM. The PLC MODEM 40 includes a power supply portion 41 and a MODEM main body portion 42. The power supply portion 41 has a rectangular outer shape, converts an A.C. commercial power source to a D.C. power source and supplies a D.C. voltage to the MODEM main body portion 42. Therefore, an attachment plug 41a for inputting the A.C. power source is fitted to one of the surfaces of the power supply portion 41, and a rectification circuit 41b is accommodated in the power supply portion 41 (see FIG. 6). The D.C. voltage acquired by the rectification circuit 41b is supplied to the MODEM main body portion 42 through a cable 43.
The MODEM main body portion 42, too, has a rectangular outer shape and incorporates therein a MODEM circuit 42a (see FIG. 6). The MODEM circuit 42a converts the digital signal to the analog signal and vice versa as described above. A connector for exchanging digital data with an information processing unit (not shown) is fitted to one of the side surfaces of the MODEM main body portion 42. The connector to be fitted includes a connector of the USB standard (called “USB connector”) 42b and a connector of the Ethernet standard (trade name of XEROX Co., DEC Co. and Intel Co.; hereinafter the same; called “Ethernet connector”) 42c. 
FIG. 6 shows a block construction of the PLC MODEM 40. The A.C. power source acquired from the attachment plug 41a is converted to the D.C. power source by the rectification circuit 41b and the D.C. voltage is supplied to the MODEM circuit 42a through the cable 43. The attachment plug 41a and the cable 43 are connected to each other by signal overlapping means 41c. The USB connector 42b and the Ethernet connector 42c are connected to the MODEM circuit 42a. 
To constitute a communication network by use of a personal computer, the attachment plug 41a is fitted into the plug socket inside home. The information processing unit (not shown) such as the personal computer is connected to the USB connector 42b or to the Ethernet connector 42c. In other words, the USB connector for inputting/outputting data of the USB standard or the Ethernet connector for inputting/outputting data of the Ethernet standard is fitted to the information processing unit. Therefore, the USB connector of the information processing unit is connected to the USB connector 42b of the MODEM main body portion 42 through a dedicated cable (shown in the drawing), or the Ethernet connector of the information processing unit is connected to the Ethernet connector 42c of the MODEM main body portion 42 through a dedicated cable (shown in the drawing).
When data is inputted from the information processing unit to the MODEM main body portion 42 under the state described above, the MODEM circuit 42a converts the data to the analog data and overlaps it with cable 43 for supplying the D.C. voltage. The data is then sent to the interior line through the attachment plug 41a. Mutual data communication becomes thus possible with other information processing unit inside the same home. Mutual communication can also be made with other homes through a distribution line.
The PLC MODEM and the information processing unit are always used as a pair. Therefore, when no spare plug socket for using the PLC MODEM exists, a plug socket for the information processing unit to be used together must be secured. Alternatively, another plug socket for branching the plug socket must be provided.